Numerical study of the magnetohydrodynamic properties of laser-produced tin plasma under external magnetic field

Laser-driven tin droplets represent the primary technique for generating extreme ultraviolet (EUV) radiation in lithography light sources. The implementation of a vertical external magnetic field is a critical strategy for reducing the ion debris produced by the tin plasma. Nonetheless, the detailed mechanisms through which the external magnetic field affects ion debris dynamics have not yet been fully elucidated. In our study, numerical simulations were conducted using the radiation hydrodynamics code FLASH to investigate the magnetohydrodynamic properties of tin plasma under an external magnetic field with a strength of 1 T. Our results reveal, for the first time, the intermediate-timescale (tens to hundreds of nanoseconds) evolution of tin plasma under an external magnetic field, providing new insights into the mechanisms of ion debris mitigation in EUV lithography systems. The magnetic field guides the plasma from the front and rear surfaces of the target to flow to both sides, forming a vortex street phenomenon, accompanied by backflow opposing the vortex street flow. Beyond the vortex region, the plasma transitions to laminar flow, where hydrodynamic instabilities between layers of varying velocities are significantly suppressed. These phenomena are recognized as significant factors to the effective trapping of tin plasma by an externally applied magnetic field in a vertical configuration.